If you are using ArchiOffice/EngineerOffice 2013, the way no-charge time slips are displayed on the standard invoices has been changed. In the older versions, when summarizing time, the employee rate would be changed to accommodate the time that was at 'no charge' (for example: $150/hour rate could change to $137.52/hour). In 2013, we changed this so all charged time is grouped together ($150/hour) and all no-charge time is grouped together ($0.0).
If you do not want to show some time slips on an invoice, you can temporarily put them on hold. After the invoice is finalized, take that time and change it to 'No-charge' and then use the Update feature to mark it as invoiced on that recently created invoice.
If you are doing some work at no charge to the client, it is highly recommended that you show that to the client on the invoice as not being charged. When clients don't see the amount of effort you expend on their projects, they will not appreciate you as much. If you later need to ask for additional fee or any favor, you can actually point to all the no-charge work done for them.